During the past several years a new flywheel technology has evolved which has resulted in a several-fold improvement in the energy density of flywheel structures while at the same time offering major advances in safety and economy. These improvements are for the most part brought about by the employment of anisotropic, filamentary materials, examples of such materials being carbon fibers, fiberglass fibers, and a new DuPont fiber known as Kevlar, all having strength-to-density properties significantly greater than the best practical steel. In addition, the filamentary composition of such materials is of significant importance in flywheel applications, since it is this property which enables the flywheel to be more readily designed for failure containment than solid steel flywheel structures previously proposed.
More particularly, it has previously been proposed that improved flywheels can be constructed in the form of wound disc structures with either "fiberglass" or steel foil as the principal structural material, such structures being described in detail in a Russian book entitled, "Inertial Energy Accumulators," by N. V. Gulia, Voronezh University Press, Voronezh, 1973. Unfortunately, such structures have had only limited success due to the hub attachment difficulty usually encountered with this type of structure. In an effort to overcome the hub attachment problem associated with wound flywheel structures, I previously proposed a circular brush flywheel configuration which utilizes radially oriented fibers or rods, such as are disclosed in my U.S. Pat. Nos. 3,698,262 and 3,737,694. On the other hand, for certain flywheel applications, it would be advantageous to have an alternative flywheel configuration which, at least in theory, appears capable of storing more energy per unit volume than this circular brush configuration, and at a reduced rotational speed for a given energy level.
The principal reason that previous attempts to build filament-wound flywheels have met with only limited success is the fact that the stress on the wound filaments varies as the square of the distance of the filaments from the center of rotation. The amount that the filament stretches is proportional to this stress; however, it is also proportional to the length of the filament, which length varies directly with the radius. Thus the amount of radial stretch actually varies with the cube of the radius. In other words, in a wound rotor having an inside radius of one-third its outside radius, the outside filaments would stretch 27 times as much as the filaments on the inside. In this situation, as has been demonstrated many times in past experiments, the flywheel breaks into many concentric rings long before the filaments have reached their breaking stress. This, of course, is true if there are no extra radial filaments in the flywheel structure to take the radial loads. On the other hand, if such extra filaments are added, then the weight of these filaments must be added when determining the energy density of the structure. This simple paradox accounts for the lack of success of the filament-wound and multi-rim flywheels previously attempted; performance typically being about 20-30% of theoretical.
One previously proposed manner of accommodating the differential stretching of the filamentary materials is to provide an elastomer matrix which acts as a spacer between the rings or filaments of a multi-ring flywheel. However, it is not clear that the elastomer can withstand the high acceleration forces occurring during flywheel operation and at the same time provide the required stretch capabilities in one direction while also providing the required stiffness in other directions. Moreover, the elastomer matrix will occupy about 30% as much space as the working filaments and thereby degrade volume, weight, and cost.